Project Summary (Public Abstract) Many lower limb amputees have difficulty maintaining balance while walking, especially in real-world situations that require quick responses to changes in terrain or surface conditions. To be certain, amputees (and others) can fall in a myriad of ways for a myriad of reasons. Unexpected slipping on a surface (e.g., due to ice, oil or other liquids on the surface) is an example of a class of falls where it is difficult to imagine a prosthetic intervention that might prevent loss of balance. Stepping on uneven terrain is a different class where we suggest a novel prosthesis might aid in balance recovery. A quarter of outdoor falls, where a wide variety of uneven terrains exist, occur in a sideways (mediolateral) direction. Maintaining balance when stepping on uneven terrain can be particularly difficult for lower limb amputees because they lack the foot-ankle muscles needed to compensate and their prostheses are passive, elastic devices whose properties cannot change or adapt. The purpose of this research is to improve the balance of lower limb amputees by providing them with an optimized prosthesis that can respond to their motor intentions and adapt to changes in terrain. To address this important issue faced by Veteran lower limb amputees, we have already developed an advanced computer modeling and simulation framework, which we will use to identify the optimal ankle properties that maximize balance recovery after a step on uneven terrain. We have also built a first generation prosthesis with variable coronal plane ankle stiffness and tested it with the help of amputees walking on a novel instrumented walkway that replicates a step on uneven terrain. The results of our preliminary tests suggest that varying coronal plane ankle stiffness can influence how an amputee controls their balance. Our proposed research has three specific aims: (1) To identify terrain-dependent coronal ankle properties that maximize balance recovery. We propose to develop a three-dimensional bipedal musculoskeletal model of human walking on uneven terrain and use it to answer a key question: What is the optimal coronal plane stiffness that maximizes balance recovery after a step on uneven terrain? (2) To identify user motor intentions before a step on uneven terrain. We propose to conduct a human subject experiment (n=20) with transtibial amputees wearing our first generation prosthesis. Subjects will be asked to walk on our instrumented walkway that replicates a single step on uneven terrain while we measure their motor intentions (surface electromyography). Using the experimental results, we will develop and test three algorithms to predict the step on uneven terrain. We hypothesize that there will be a difference in accuracy among the three algorithms. We will use the results to specify a control law that best predicts a step on uneven terrain for use with our novel prosthesis. (3) Determine if a novel prosthesis optimized for balance recovery and controlled by user motor intentions can improve the recovery from a step on uneven terrain when compared to the amputee's as- prescribed prosthesis. To achieve this aim, we propose to build a second generation prosthesis that incorporates the results from Specific Aims 1 and 2, as well as the lessons learned from our first generation prosthesis. We then propose to conduct a human subject experiment with transtibial amputees (n=20) that will measure their balance recovery after a step on uneven terrain. We hypothesize that our novel prosthesis will improve the recovery of balance after a step on uneven terrain when compared to the amputee's as-prescribed prosthesis. Expanding the terrain over which Veteran amputees can confidently walk is where the VA should be: at the forefront of prosthetic technology and prescription practice that advances amputee care.